A known internal gear serving as a gear for a machine element is used for, for example, a high-efficiency reduction device. Such an internal gear is manufactured by a form rolling process in order to reduce manufacturing man-hours as disclosed in JP3947204B (hereinafter referred to as Patent reference 1). For example, according to a manufacturing method of the internal gear disclosed in Patent reference 1, a cylindrical work is inserted into an inside of a container having a high rigidity, and a die is pressed against the work while rotating inside of the work. Because the die is rotationally driven and is pressed against the work, the work and the container are rotationally driven by the rotation of the die.
According to Patent reference 1, recessed grooves having a same number as the number of teeth of the internal gear that is to be manufactured are positioned on an inner circumferential surface of the work at a predetermined interval in order to synchronize the work with the die.
Accordingly, because the work is fixed at the inside of the container having the high rigidity, a circularity of the work is secured. Thus, because an influence of an unbalanced load caused by processing is not remained, the form rolling process in accordance with a large deformation is effectively performed.
According to Patent reference 1, the manufacturing method of the internal gear in general is described. However, a specific tooth profile of the internal gear and a specific manufacturing method for obtaining the specific tooth profile are not specifically described.
A known internal gear includes various tooth profiles in accordance with positions to be used. For example, in order to obtain the internal gear that greatly reduces the rotational speed of the gear and that has a high-load capability, for example, the internal gear used for a hypocycloid reduction gear, the internal gear is desired to include teeth each having a tooth profile that makes the internal gear be unlikely to cause a tooth jump. A die tool that satisfies a predetermined requirement is desired to be used in order to obtain the internal gear that greatly reduces the rotational speed of the gear and that has the high-load capability.
A need thus exists for an internal gear and a manufacturing method thereof with a die which is not susceptible to the drawback mentioned above.